Cellular Biology

Enzymes and Metabolism

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Metabolism, the totality of all the chemical reactions that occur in an organism, is a complex tapestry of chemical change woven together by the action of enzymes.  In organisms, chemical reactions require an initial input of energy called the energy of activation. Some reactions can often absorb enough ambient energy from the environment to move forward and occur spontaneously. This process is often slow. Enzymes speed things up.  An enzyme is a catalytic protein that speeds up a chemical reaction while not being consumed in the chemical reaction.  All the chemical reactions of life are enzyme-mediated.

As proteins, enzymes are complex macromolecules with a specific three-dimensional shape called a conformation. At a specific location of the molecule is what is known as the enzyme's active site. This is the location to which reactants of the chemical reaction will bind. These reactants are the substrate on which the enzyme acts to aid in the formation of product. It is important to note that enzymes are substrate-specific meaning that for each enzyme there is a specific substrate that will bind to its active site. This specificity has often been called a lock-and-key mechanism likened to the fit of jigsaw puzzle pieces. This view of the formation of an enzyme-substrate complex however is now believed to be inadequate. The binding of substrate to the active site is now believed to be an induced fit. As the substrate binds to the active site of the enzyme, the enzyme undergoes a slight conformational shape change so that its "grasps" the substrate tightly.

How do enzymes catalyze reactions? As the enzyme-substrate complex forms several things may occur that allow the enzyme to lower the activation energy of the reaction thus increasing its rate. First, the enzyme positions the reactants favorable for the impending chemical change. This may stress bonds that eventually will be broken or may orient two reactants close together that will form a new bond. Secondly, the active site likely creates a microenvironment that favors the reaction. The amino acid side chains that protrude into the active site may alter its pH, charge, level of hydrophobicity, or other environmental variables. Finally, the enzyme could act as a temporary participant in the reaction where its own bonds are altered. However, by the end of the reaction the enzyme is unchanged and available to be used again.

Enzymes come in many types based on the substrate on which they act. There are lipases that act on lipids, proteases that act on proteins, nucleases that act on nucleic acids, and so on. An enzyme is often named based on the name of its substrate. For example, sucrase acts on sucrose. Sucrase can help break down sucrose into its monomers, glucose and fructose, or it can join those same monomers. The variety of enzymes in an organism is huge due to the breadth of metabolic processes that occur in the organism. Breaking down glucose, glycolysis, requires ten enzymes, one for each of its ten steps. Regardless of the metabolic process, an enzyme is at hand.

Enzymes mediate the vibrant tapestry of metabolism. The rich colors of life cannot be dulled by sluggish chemical processes. By lowering the initial energy costs, enzymes keep metabolism moving at the pace needed for life.

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